1. Field of the Invention
The invention relates to the manufacture of composite pigments containing aluminum hydroxide, and their use for improving the pigment light-scattering efficiency in coatings, plastics, paper and laminates.
2. Technological Background of the Invention
Inorganic pigments, and particularly titanium dioxide pigments, are often incorporated into various matrices as whiteners, tinting agents or opacifiers. Owing to its high refractive index, titanium dioxide scatters light particularly efficiently and is therefore the most important white pigment for applications in paints and coatings, plastics, paper and fibers. The light-scattering efficiency declines if the titanium dioxide particles are distributed in the matrix at a distance of less than roughly half the wavelength of light from each other, i.e. roughly 0.20 to 0.25 μm. The light-scattering efficiency is typically measured with the help of the hiding power or the tinting strength of the titanium dioxide pigment in the matrix.
On the other hand, titanium dioxide is a significant cost factor, and a search has been in progress for some time to find possibilities for reducing the quantity of titanium dioxide used, without having to accept significant losses of hiding power.
Savings are possible by combining titanium dioxide particles with suitable fillers that, as so-called “extender particles”, are intended to act as spacers for the TiO2 particles. The known methods include both simple blending of the components and also the combination of the TiO2 particles with the extender particles by means of a precipitated binder, or in-situ precipitation of the extender on the surface of the titanium dioxide particles. Furthermore known are methods for distributing the fine titanium dioxide particles on coarser extender particles.
Only selected publications from the extensive available prior art are cited below.
WO 1999/035193 A1, for example, describes the manufacture of a pigment blend consisting of titanium dioxide and an inorganic extender (“spacer”), such as SiO2 or CaCO3, for use in the manufacture of paper.
DE 10 057 294 C5 discloses a pigment blend consisting of titanium dioxide and talcum for use in base decor paper.
EP 0 861 299 B1 discloses a titanium dioxide pigment that is coated with inorganic nanoparticles, such as colloidal silica, and a layer consisting of inorganic oxides, such as aluminum, silicon or zirconium oxide, where the inorganic oxide layer is either located between the titanium dioxide surface and the nanoparticles, or forms the outer coating. The inorganic nanoparticles are SiO2, Al2O3 or CaCO3.
DE 10 2006 012 564 A1 discloses titanium dioxide pigment particles, on whose surface hollow microspheres and an aluminum oxide/aluminum phosphate coating are located.
In the method according to EP 0 956 316 B 1, pigment particles and precipitated calcium carbonate (PCC) are mixed together in an aqueous phase, so as to produce a composite pigment where the carbonate particles, with a particle size of 30 to 100 nm, are attached to the surface of the pigment particles. The composite pigment contains 30 to 90% by weight precipitated calcium carbonate.
DE 1 792 118 A1 discloses the in-situ precipitation of calcium carbonate in a titanium dioxide pigment suspension by mixing of a calcium chloride and a sodium carbonate solution, where one of these solutions contains titanium dioxide pigment. This results in composite calcium carbonate/titanium dioxide particles.
WO 2000/001771 A1 discloses a composite pigment that contains inorganic particles having a particle size of roughly 1 to 10 μm, to the surface of which titanium dioxide pigment particles are attached as a result of their opposite surface charge. Production takes place in an aqueous phase. The inorganic particles are selected from common extenders, such as kaolin, clay, talcum, mica or carbonates.
Patent application WO 2014/000874 A1 discloses a composite pigment that contains titanium dioxide and a particulate material as an extender, as well as calcium carbonate that is precipitated in the production process. The composite particles are manufactured in a combined process of dispersion and precipitation. The composite pigment is suitable for use in coatings, plastics and laminates.
Patent application EP 1 3005 813.4 (filed on Dec. 13, 2013) discloses a composite pigment that contains inorganic pigment particles, particularly titanium dioxide, and precipitated particulate calcium phosphate. The composite pigment is suitable for use in coatings, plastics, and particularly in paper and laminates.